This invention relates to the removal of sulfur from a hydrocarbon feedstock, particularly the removal of extremely small quantities of thiophene sulfur.
Generally, sulfur occurs in petroleum and syncrude stocks as hydrogen sulfide, organic sulfides, organic disulfides, mercaptans, also known as thiols, and aromatic ring compounds such as thiophene, benzothiophene and related compounds. The sulfur in aromatic sulfur-containing ring compounds will be herein referred to as "thiophene sulfur".
Conventionally, feeds with substantial amounts of sulfur, for example, those with more than 10 ppm sulfur, are hydrotreated with conventional catalysts under conventional conditions, thereby changing the form of most of the sulfur in the feed to hydrogen sulfide. Then the hydrogen sulfide is removed by distillation, stripping or related techniques. Such techniques can leave some traces of sulfur in the feed, including thiophenic sulfur, which is the most difficult type to convert.
Such hydrotreated naphtha feeds are frequently used as feed for catalytic dehydrocyclization, also known as reforming. Some of these catalysts are extremely sulfur sensitive, particularly those that contain zeolitic components. Others of these catalysts can tolerate sulfur in the levels found in typical reforming feeds.
One conventional method of removing residual hydrogen sulfide and mercaptan sulfur is the use of sulfur sorbents. See for example U.S. Pats.4,204,997 and 4,163,708, both by R. L. Jacobson and K. R. Gibson. The concentration of sulfur in this form can be reduced to considerably less than 1 ppm by the use of the appropriate sorbent and conditions, but it is difficult to remove sulfur to less than 0.1 ppm or to remove any residual thiophene sulfur. See for example U.S. Pat. 4,179,361 by M. J. Michlmayr, and particularly Example 1 in that patent. In particular, very low space velocities are required, to remove thiophene sulfur, requiring large reaction vessels filled with sorbent, and even with these precautions, traces of thiophene sulfur can get through.
It would be advantageous to have a process to remove most sulfur, including thiophene sulfur, from a reforming feedstream.